1. Field of the Invention
This invention relates to nanotube (NT) growth of Carbon and other materials such as Germanium, Boron, Boron-Nitride, Boron-Carbide, BiCjNk, Silica and Silica-Carbide, and more particular to a low-power approach to growing nanotubes.
2. Description of the Related Art
Carbon nanotubes (CNTs) have stimulated a great deal of interest in the microelectronic and other industries because of their unique properties including tensile strengths above 35 GPa, elastic modulus reaching 1 TPa, higher thermal conductivity than diamond, ability to carry 1000× the current of copper, densities below 1.3 g/cm3 and high chemical, thermal and radiation stability. CNTs have great promise for devices such as field effect transistors, field emission displays, single electron transistors in the microelectronic industry, and uses in other industries. Commercialization of CNTs will depend in large part on the ability to grow and network CNTs on a large cost-effective scale without compromising these properties.
As illustrated in FIG. 1, a CNT 10 is a hollow cylindrical shaped carbon molecule. The cylindrical structure is built from a hexagonal lattice of sp2 bonded carbon atoms 12 with no dangling bonds. The properties of single-walled nanotubes (SWNTs) are determined by the graphene structure in which the carbon atoms are arranged to form the cylinder. Multi-walled nanotubes (MWNTs) are made of concentric cylinders around a common central hollow.
CNTs are commonly grown using several techniques such as arc discharge, laser ablation and chemical vapour deposition (CVD). In CVD the growth of a CNT is determined by the presence of a catalyst, usually a transition metal such as Fe, Co or Ni, which causes the catalytic dehydrogenation of hydrocarbons and consequently the formation of a CNT. CVD generally produces MWNTs or SWNTs of relatively poor quality due mostly to the poorly controlled diameters of the nanotubes. However, CVD is relatively easy to scale up and can be integrated with conventional microelectronic fabrication, which favors commercialization.
The way in which nanotubes are formed at the atomic scale is not precisely known. The growth mechanism is still a subject of scientific debate, and more than one mechanism might be operative during the formation of CNTs. As shown in FIGS. 2a and 2b, a catalyst 20 is deposited on a support such as silicon, zeolite, quartz, or inconel 22. At elevated temperatures, exposure to a carbon containing gas causes the catalyst to take in carbon, on either the surfaces, into the bulk, or both. This thermal diffusion process of neutral carbon atoms occurs at energies of a few electronvolts (eV). A precursor to the formation of nanotubes and fullerenes, C2, is formed on the surface of the catalyst. From this precursor, a rodlike carbon 24 is formed rapidly, followed by a slow graphitization of its wall. The CNT can form either by ‘extrusion’ (also know as ‘base growth’ or ‘root growth’) shown in FIG. 2a, in which the CNT grows upwards from the catalyst that remains attached to the support, or the particles can detach from the substrate and move at the head of the growing nanotube, labelled ‘tip-growth’, as shown in FIG. 2b. Depending on the size of the catalyst particle either SWNT or MWNT are grown. A typical catalyst may contain an alloy of Fe, Co or Ni atoms having a total diameter of 1 to 100 nm (on the order of 1,000 atoms for 1 nm diameter of catalyst).
The application of thermal energy or heat is essential to stimulate the growth mechanism of CNTs. Heat is required to break the hydrocarbon molecules in the carbon containing gas upon colliding with the catalyst so they attach to the catalysts. Heat is required to transport these carbon atoms via diffusion processes to the interface of the catalyst and the carbon nanotubes to obtain higher growth rates. Heat is required for the CNT to attach the carbon atoms quickly for fast growth. The thermal energy must be controlled to provide sufficient heating to stimulate these growth processes without melting the catalyst of breaking the CNT. Typically heating is provided by induction, plasma discharge, substrate or wall heating. The power consumption required by these methods of indirect heating of the catalyst is a significant factor in the manufacturing cost.
As shown in FIG. 3, to synthesize CNTs 24 using CVD the support 22 and catalytic material 20 are placed inside an environmentally-controlled chamber 32. The sample is heated until the temperature is great enough (400° C.) that the introduction of hydrogen along with a buffer gas (Argon) “reduces” (removes the oxide) the particle. A plurality of gas feeds 34 introduce a process gas including a mixture of a carbon-containing growth gas 36, typically a hydrocarbon CxHy such as Ethylene (C2H4), Methane (CH4), Ethanol (C2H5OH), or Acetylene (C2H2) or possibly a non-hydrocarbon such as carbon-monoxide (CO), an inert buffer gas 38 such as Argon (Ar) to control pressure inside the chamber and prevent released hydrogen atoms from exploding and possibly a scrubber gas 40 such as H2O or O2 to periodically or continuously clean the surface of the catalyst. An energy source 42 such as induction, plasma discharge, substrate or wall heating provides the energy necessary (e.g. a few eV) to heat the catalyst to a temperature which allows it to ‘crack’ the hydrocarbon molecules into reactive atomic carbon 44 upon colliding with the catalyst, to heat the catalyst to increase the transport of carbon to the catalysts/CNT interface and to heat the CNT itself. The reactive carbon 44 is absorbed into the surface of catalytic material 20 causing the CNT to grow from the same catalytic surface. A pump system 46 including a vacuum and/or pressure pump controls the pressure inside the chamber to produce conditions both conducive to absorption of carbon atoms into the catalytic material and growth of CNTs from the catalytic material. A number of electrical ports 48 are provided to accommodate pressure sensors, thermocouples and the like to monitor conditions inside the chamber.
As shown in FIGS. 4a and 4b, CVD can be used to synthesize an array of vertically aligned CNTs 50 between a Si substrate 52 and a metal thin-film 54, suitably nickel, via a lift-off process. The thin-film is formed over Fe particles 56 on substrate 52 that serve as catalysts. The CVD process initiates nanotube growth that ‘lifts’ thin-film 54 off of the substrate. The fabrication of three-dimensional networks of CNTs with controlled orientation will be essential for building large-scale function devices integrated with microelectronics circuits. Bingqing Wei et al. “Lift-up growth of aligned carbon nanotube patterns” Applied Physics Letters Volume 77, Number 19 6 Nov. 2000 and JacquelinMerikhi et al. “Sandwich growth of carbon nanotubes” Diamond & Related materials 15 (2006) pp. 104-106.